generate/rate_comprehension/conversion.py: convert_time(value, from_unit, to_unit) -> Fraction for known time units (minute<->hour), exact rational (fractions.Fraction), NO floats. is_convertible gates it. 30 minute -> Fraction(1,2) hour; non-time/non-convertible (minute<->mile, dollar<->hour) raises ConversionError so the caller still refuses rate_unit_mismatch. Tiny: only minute<->hour in v1 (length/currency/compound deferred). 6 tests incl. never-a-float guard.
